Prior art microwave applicators with some similarities to the present invention are described in U.S. Pat. No. 5,828,040 and its European counterpart EP 0 746 182.
The particular single hybrid mode applicator in the referenced prior art solve a major problem of still earlier prior art, namely that of uneven heating and that of excessive edge overheating. Uneven heating is evidenced by a patchy and quite unpredictable heating pattern with hot and cold spots (caused by multimode action). Edge overheating typically occurs for loads having a high permittivity, such as typical compact food items. Edge overheating is caused by strong electric horizontal field components which are then parallel to the major edges of the food item.
The particular type of propagating hybrid mode in the applicator of the above prior art is characterized by very low vertically (z-direction) directed real impedance. This results in low horizontal (x- and y-direction) electric field strengths in relation to those of perpendicularly (z-directed) impinging plane waves. By the choice of a TEy hybrid mode, the y-directed electric field component in the applicator becomes zero, which is still more advantageous since edge overheating of y-directed load edges will then not occur. It should be noted that the feed orientation determines if the mode becomes a TEy or a TEx mode. The edge overheating effect is a non-resonant microwave diffraction phenomenon caused by an impinging E-field component parallel to the edge. This phenomenon is insensitive to the direction of impingement, as long as the resulting propagation in the wedge is away from its edge.
The particular low impedance applicator mode preferably has the lowest possible horizontal index (i.e. equal to 1) in the direction of transport of the load, since microwave leakage in that direction from the applicators is then minimized. Thereby, interaction (cross-coupling) between consecutive applicators in this direction is minimized, which reduces the complexity of the microwave choking structures at the tunnel end. With the load transport in the y-direction, the heating pattern of each individual applicator in moving loads therefore becomes striped. This is compensated for by a sideways (i.e. in the x-direction) staggering of consecutive applicators or applicator rows.
The particular low impedance TEy mode has a tendency to create a trapped surface wave mode (a so-called Longitudinal Section Magnetic mode, or LSM-mode) in the region including the underside of the load items and the metallic bottom structure of the tunnel. Although such modes result in a favorable heating from below in typical food items of about 15 mm or more in height, there is a problem when several staggered applicators are used in that a significant part of the heating pattern is determined by the x-directed standing LSM waves between the sidewalls of the tunnel oven, and not only by the fields of the individual applicators.
When the above-mentioned TEy mode is employed, there may be a tendency of both spreading-out of the applicator fields in the x-direction, and of cross-coupling between applicators. By cross-coupling, it is means an unwanted power transfer between adjacent applicators, either by direct coupling or by LSM mode coupling through the load region. The prior art referenced above does not provide any remedy to these imperfections.
According to the above-referenced prior art, the preferred embodiments comprise slot feed in the top of the applicator sidewalls, the applicator being designed for the TEy11 or TEy21 modes. However, there are cases when larger applicator openings are preferred, in order to achieve a lower power flux density to the load items without any need for reducing the output power of each microwave generator (magnetron). In order to successfully design microwave applicators for higher modes, e.g. TEy31 or TEy51 or TEy71 modes, other microwave feeding means become necessary.
If the tunnel height is large, there will be an increased likelihood of microwave leakage through the tunnel ends into the surrounding ambient. For fixed tunnel heights, it is then possible to use various kinds of prior art chokes, such as delay lines, quarter-wave chokes and chokes which act by mode mismatching. Absorbing media may also be used for this purpose. Such chokes or absorbers are normally only applied to the horizontal surfaces (top and bottom) of the tunnel opening, but may also be used at the vertical side walls in the tunnel opening and choking region. However, if the tunnel height is to be variable, prior art choke structures in the vertical walls become very difficult to implement.
Another set of problems with the prior art is related to the overall height of the applicator plus the tunnel underneath. This is addressed in some detail in the referenced prior art, where the “effective height” in the system is a quite sensitive parameter. In order to achieve an acceptably low reflection factor (weak mismatching) of the system, constraints must be put on the “effective height” as well as on the permittivity of the load. In conjunction with this, it is to be noted that Brewster mode conditions are considered in the prior art to be the most desirable. Neither quarter-wave resonant modes, nor zero order modes are addressed.